1. Technical Field
The present invention relates to a motor control device and a motor control method which can optimize efficiency according to a rotation speed of an electric motor.
2. Description of Related Arts
In the past, a brushless motor that allows speed control in a wide range has been used as a fan motor. As described in JP 2002-191186 A described below, the brushless motor includes a plurality of Hall elements, and detects a rotation position of a rotor, using a signal output from the Hall elements.
The energization pattern of the stator coils in the brushless motor is determined according to a rotation position of the rotor. The energization pattern is a pattern indicating the stator coil connected to the power supply according to the rotation position of the rotor as well as a direction of the current that flows through the stator coil connected to the power supply. The stator coils are energized according to a predetermined energization pattern depending on the rotation position of the rotor.
However, the energization pattern of the stator coils changes simultaneously with the change of a signal output from one of the plurality of Hall elements. Usually, timing of change of the signals output from the Hall elements becomes quick as the rotation speed of the rotor decreases.
In the past brushless motor, therefore, as the rotation speed decreases, the timing of the position detection of the rotor by the Hall elements becomes quicker, and the energization pattern switches more rapidly as compared with the optimum timing. Ina low rotation speed range, therefore, it becomes difficult for the rotor to use effectively a magnetic force generated by the stator coils so that efficiency lowers, and power consumption with respect to the output increases.
For compensating for deviation of detection timing of the rotor position that occurs in a low rotation speed range, it is envisaged to shift the arrangement of the Hall elements in the rotation direction of the rotor by an amount corresponding to the timing deviation. In this manner, however, the efficiency of the fan motor operating at a variable speed cannot be always optimized according to the rotation speed.